Method for solidifying aqueous radioactive wastes for non-contaminating storage

Abstract

Method for solidifying high and medium radioactivity and/or actinide containing aqueous waste concentrates or fine-grained solid wastes suspended in water for final noncontaminating storage. The waste concentrates or suspensions are set, by evaporation, to a water content in the range between 40 and 80 percent by weight, and a solid content whose metal ion and/or metal oxide component lies between 10 and 30 percent by weight of the evaporate being formed. The pH of the evaporate is set to between 5 and 10. The evaporate is kneaded with a clay-like substance containing a small quantity of cement or such a clay-like substance or mixture of a clay-like substance with a small quantity of cement containing an additive for suppressing the volatility of the alkalis or alkali earths and/or an additive for suppressing the volatility of any decomposing anions which may be present in the evaporate from the group including sulfate, phosphate, molybdate and uranate ions. The kneading occurs at a weight ratio range of evaporate to clay-like substance of 1:1 to 2:1. Molded bodies are produced from the resulting kneaded mass. The molded bodies are then heat treated including drying at temperatures between room temperature and about 150 DEG C., calcining at temperatures up to about 800 DEG C. and subsequently firing at temperatures between 800 DEG C. and 1400 DEG C. to form practically undissolvable mineral phases. The molded bodies of fired mineral phases are enclosed on all sides in a dense, continuous ceramic or metallic matrix.

What is claimed is: 
     
       1. Method for solidifying high radioactivity aqueous waste concentrates, medium radioactivity aqueous waste concentrates, actinide containing aqueous waste concentrates, or suspensions of fine-grained solid wastes suspended in water, which concentrates or suspensions contain a metal ion and/or metal oxide, for final noncontaminating storage in which the waste concentrates or the suspensions are subjected together with an absorbing and/or hydraulically binding inorganic material, to a ceramic firing process so as to produce a solid sintered body, comprising the steps of: (a) treating the waste concentrates or suspensions by evaporation, to form an evaporate, to a water content in the range between 40 and 80 percent by weight and a solid content whose metal ion and/or metal oxide concentration lies between 10 and 30 percent by weight of the evaporate being formed, and adjusting the pH of the evaporate to between 5 and 10;   (b) kneading the evaporate obtained from step (a) with a clay-like substance containing a small quantity of cement or mixture of a clay-like substance with a small quantity of cement containing an additive for suppressing the volatility of alkali metals or alkaline earth metals which may be present in the evaporate and/or an additive for suppressing the volatility of any decomposable anions which may be present in the evaporate selected from the group consisting of sulfate, phosphate, molybdate and uranate ions, at a weight ratio range of evaporate to clay-like substance of 1:1 to 2:1, said clay-like substance being at least one substance selected from the group consisting of pottery clays, stoneware clays, porcelain clay mixtures and kaolins;   (c) producing molded bodies from the kneaded mass obtained from step (b);   (d) heat treating the molded bodies, including drying at temperatures between room temperature and 150° C., calcining at temperatures of about 150° to 800° C., and subsequently firing at temperatures between 800° and 1400° C. to form practically undissolvable mineral phases having a chemical composition corresponding approximately to that of natural, stable minerals or rocks; and   (e) enclosing the molded bodies containing the fired mineral phases on all sides in a dense, continuous ceramic or metallic matrix.   
     
     
       2. The method as defined in claim 1, in which the molded bodies of step (d) are comminuted to a grain size range of about 1 to 10 mm before being enclosed in the matrix of step (e). 
     
     
       3. The method as defined in claim 1 wherein the kneading of step (b) is effected with a mixture of 10 parts by weight of a clay-like substance and 1 to 2 parts by weight of a cement containing 20 to 30 weight percent SiO 2  and 40 to 70 weight percent CaO, and said molded bodies of step (c) are allowed to harden, and are subsequently surface decontaminated with water before step (d). 
     
     
       4. The method as defined in claim 1 wherein said clay-like substance contains SiO 2  in a range from about 45 to 70 percent by weight and Al 2  O 3  in a range from about 15 to 40 percent by weight and has a loss due to heating which lies in the range from about 5 to 15 percent by weight. 
     
     
       5. The method as defined in claim 1 wherein the additive for suppressing the volatility of the alkali metals and alkaline earth metals comprises about 1 to 3 parts by weight TiO 2  powder compared to 20 parts by weight clay-like substance. 
     
     
       6. The method as defined in claim 1 wherein the additive for suppressing the volatility of the alkali metals and alkaline earth metals comprises about 1 to 5 weight percent TiO 2  with respect to the kneaded mass obtained from process step (b). 
     
     
       7. The method as defined in claim 1 wherein the additive for suppressing the volatility of sulfate, molybdate and/or uranate comprises 1 to 5 weight percent BaO with respect to the kneaded mass obtained from step (b). 
     
     
       8. The method as defined in claim 1 wherein the additive for suppressing the volatility of phosphate comprises about 2 to 10 weight percent MgO with respect to the kneaded mass obtained from step (b). 
     
     
       9. The method as defined in claim 1 wherein the additive for suppressing the volatility of phosphate comprises about 2 to 10 weight percent BeO or ground natural beryllium with respect to the kneaded mass of step (b). 
     
     
       10. The method as defined in claim 1 wherein the continuous matrix comprises at least one cement selected from the group consisting of portland cement, iron portland cement, shaft furnace cement, trass cement, oil shale cement, alumina cement and mixtures thereof. 
     
     
       11. The method as defined in claim 1 wherein the continuous matrix comprises a fired ceramic produced from (1) at least one clay-like substance selected from the group consisting of pottery clays, stoneware clays, porcelain clay mixtures, and kaolin and (2) at least one cement selected from the group consisting of portland cement, iron portland cement, shaft furnace cement, trass cement, oil shale cement, and alumina cement in a weight ratio range of clay-like substance to cement of 10:1 to 4:1. 
     
     
       12. The method as defined in claim 1 wherein the continuous matrix is made of a copper-zinc alloy. 
     
     
       13. The method as defined in claim 1 wherein the continuous matrix is made of a copper-tin alloy. 
     
     
       14. The method as defined in claim 1 wherein the continuous matrix is made of lead or a lead alloy having a lead content of more than 50 percent by weight. 
     
     
       15. The method as defined in claim 1 wherein the adjusting of the pH is effected by the addition of a strongly alkali solution. 
     
     
       16. The method as defined in claim 1 which comprises adjusting the pH of said evaporate by denitrating. 
     
     
       17. The method as defined in claim 16 wherein the denitrating is effected with formaldehyde. 
     
     
       18. The method as defined in claim 1 which comprises adjusting the pH of the evaporate by denitrating with formic acid. 
     
     
       19. The method as defined in claim 1 which further comprises measuring the water and NO x  content of gases given off during the drying and calcining stages of step (d) and varying the time and temperature of the drying and calcining stages as a function of the measured water and NO x  content. 
     
     
       20. Method for solidifying high radioactivity aqueous waste concentrates, medium radioactivity aqueous waste concentrates, actinide containing aqueous waste concentrates, or suspensions of fine-grained solid wastes suspended in water, which concentrates or suspensions contain a metal ion and/or metal oxide, for final noncontaminating storage in which the waste concentrates or the suspensions are subjected together with an absorbing and/or hydraulically binding inorganic material, to a ceramic firing process so as to produce a solid sintered body, comprising the steps of: (a) treating the waste concentrates or suspensions by evaporation, to form an evaporate, to a water content in a range between 40 and 80 percent by weight and a solid content whose metal ion and/or metal oxide concentration lies between 10 and 30 percent by weight of the evaporate being formed, and adjusting the pH of the evaporate to between 5 and 10;   (b) kneading the evaporate obtained from step (a) with a clay-like substance containing an additive for suppressing the volatility of alkali metals or alkaline earth metals which may be present in the evaporate and/or an additive for suppressing the volatility of any decomposable anions which may be present in the evaporate selected from the group consisting of sulfate, phosphate, molybdate and uranate ions, at a weight ratio range of evaporate to clay-like substance of 1:1 to 2:1, said clay-like substance being at least one substance selected from the group consisting of pottery clays, stoneware clays, porcelain clay mixtures and kaolins;   (c) producing molded bodies from the kneaded mass obtained from step (b);   (d) heat treating the molded bodies, including drying at temperatures between room temperatures and 150° C., calcining at temperatures of about 150° to 180° C., and subsequently firing at temperatures between 800° and 1400° C. to form practically undissolvable mineral phases having a chemical composition corresponding approximately to that of natural, stable minerals or rocks; and   (e) enclosing the molded bodies containing the fired mineral phases on all sides in a dense, continuous ceramic or metallic matrix.   
     
     
       21. The method as defined in claim 1, wherein said natural, stable mineral is nepheline. 
     
     
       22. The method as defined in claim 1, wherein said natural, stable mineral is anorthite. 
     
     
       23. The method as defined in claim 1, wherein said natural, stable mineral is noselite. 
     
     
       24. The method as defined in claim 1, wherein said natural, stable mineral is sodalite.